Field of the Invention
The present invention is related to fine wire bonding and the operation of fine wire bonding machines employed in the manufacture of semiconductor devices. More particularly, the present invention is related to a novel method of forming fine wire interconnection between terminals on an integrated circuit and the lead out pads on a substrate or package.
Fine wire bonding machines have been employed to connect wire smaller than an average human hair between electrodes and terminals of semiconductor devices. There are numerous discrete semiconductor devices and integrated circuits where the shape of the interconnecting wire need not conform to an exact predetermined shape and height. As a general rule, if the interconnecting wires are being made between points which are below sixty one thousandths of an inch (60 mils) apart and the wires are not close to each other the interconnections could tolerate variations in loop height, bends in the interconnection wire and slack in the interconnection wire.
Manually operated wire bonding machines such are those described in U.S. Pat. Nos. 3,543,988 and 3,643,321 teach apparatus designed to make fine wire interconnections. These and other prior art wire bonding machines were provided with means for adjusting the height the bonding tool is raised after making a first bond in order to provide a loop in the interconnecting wire. Most of the prior art bonding machines raised the bonding tool an excessive amount causing a great excess of wire to be pulled out of the bonding tool. This great excess of wire caused irregular loops to be formed. The loops could be too high or too low. The loop could sag and touch at intermediate points. The loops could bend sideways or bow and touch adjacent wires.
Automatic bonding machines such as Model 1418/1419 made by Kulicke and Soffa Industries, Inc. of Horsham, Pa. are provided with microprocessor controls which enable the path of the bonding to be controlled precisely. Even though automatic wire bonders have been commerically available for several years, heretofore it has been impossible to make repeatable slack free interconnection between conductive pads. Further, automatic wire bonders are operated at speeds where four interconnection wires are made in one second. It is impossible to visually inspect each interconnection wire and to exert some manual input to perfect the form of the interconnection wires.
Prior art automatic wire bonders are programmable to enable the bonding tool to be moved in virtually any predeterminable path. Numerous paths have been tried for many years, however, the paths employed heretofore did not result in repeatable slack free interconnecting wires. When the distance between bonding points on conductive pads exceeded eighty one thousandths of an inch it was virtually impossible to control the shape of the interconnection wires.
Some of the causes of inconsistent interconnection wires are known. For example, a bend or a bow in an interconnecting wire can be caused by excessive wire in the interconnection. Excessive wire can be caused by wire which is work harden by rubbing on a tool or by taking the shape of a wire spool on which the wire is wound such that the bow or bend is inherently in the wire and is not removed during the bonding operation which forms the interconnection. When an excessive length of wire is paid out of a capillary bonding tool and the bonding tool is moved relative to the wire to reduce the exposed length of wire, the bonding tool is capable of frictional engagement with the wire to bend or bow the wire thus leaving different lengths or wire exposed for making the interconnection.
It has been suggested that the desired length of wire for making the interconnection be paid out of a capillary bonding tool and the wire clamped relative to the bonding tool and then moving the bonding tool to the second conductive pad to make the second bond. This method of bonding assures the correct length of wire for each interconnection but has not been able to produce consistently form loops free of slack, bends, bows or distortion.
Large integrated circuits have introduced a new problem which affects the consistency of interconnection wires. As the number of connection points on the perimeter of the integrated circuit chips has increased, the number of outer leads or outer connection points has increased. To prevent crowding the outer leads they have been moved further away from the pads or connection points on the integrated circuit chips. For example, a square or rectangular chip is surrounded by an array of outer leads three to five mils wide and separated from each other by three to five mils. The array of outer leads in usually arranged in the same square or rectangular pattern which is larger than the chips. Accordingly, there is only one pad at the center of each side of the chip which is axially aligned with the outer lead or finger. The distance of the other interconnection is also greater than the interconnection which is axially aligned. The interconnection wire which connects the conductive pad at the corner of the chip to the finger or lead in the corner of the array of outer leads not only is the longest, but crosses over the outer lead at an angle. When the bonding tool is attempting to make a second bond on this corner outer lead a portion of the wire touches the side of the outer lead finger which exerts a side force great enough to move the wire before a bond can be made. When such side forces are present interconnection wires have been distorted and produce unacceptable devices. While the problem has existed for many years, only recently have the dimension become so critical that unacceptable interconnection wires could not be produced using manual or automatic wire bonders.